Branden Thomas Allen

The Study of TeV Variability and the Duty Cycle of Mrk 421 from 3 Yr of Observations with the Milagro Observatory

TeV-flaring activity with timescales as short as tens of minutes and an orphan TeV flare have been observed from the blazar Markarian 421 (Mrk 421). The TeV emission from Mrk 421 is believed to be produced by leptonic synchrotron self-Compton (SSC) emission. In this scenario, correlations between the X-ray and the TeV fluxes are expected, TeV orphan flares are hardly explained, and the activity (measured as duty cycle) of the source at TeV energies is expected to be equal to or less than that observed in X-rays if only SSC is considered. To estimate the TeV duty cycle of Mrk 421 and to establish limits on its variability at different timescales, we continuously observed Mrk 421 with the Milagro observatory. Mrk 421 was detected by Milagro with a statistical significance of 7.1 standard deviations between 2005 September 21 and 2008 March 15. The observed spectrum is consistent with previous observations by VERITAS. We estimate the duty cycle of Mrk 421 for energies above 1 TeV for different hypotheses of the baseline flux and for different flare selections and we compared our results with the X-ray duty cycle estimated by Resconi et al. The robustness of the results is discussed.

Spectroscopic Follow-up of X-Ray Sources in the ChaMPlane Survey: Identification of a New Cataclysmic Variable

We present a multi-object optical spectroscopy follow-up study of X-ray sources in a field along the Galactic plane (l = 327fdg42, b = 2fdg26) which is part of the Chandra Multi-wavelength Plane survey (ChaMPlane). We obtained spectra for 46 stars, including 15 likely counterparts to X-ray sources, and sources showing an Hα color excess. This has led to the identification of a new cataclysmic variable (CV), CXOPS J154305.5-522709, also named ChaMPlane Bright Source 7 (CBS 7), and we identified eight X-ray sources in the field as active late-type stars. CBS 7 was previously studied in X-rays and showed a hard spectrum and two periods: 1.22 ± 0.08 hr and 2.43 ± 0.26 hr. We present here clear evidence that the source is a CV through the detection of H, He I, and He II emission lines in its optical spectrum. The hard X-ray spectrum and the presence of the He II λ4686 in emission with a large equivalent width suggest a magnetic CV. The near-infrared counterpart is significantly variable, and we found a period consistent with the longest X-ray period at 2.39 ± 0.05 hr but not the shortest X-ray period. If this period is the orbital period, this would place the system in the CV period gap. The possible orbital period suggests a dM4 ± 1 companion star. The distance is then estimated to be ~1 kpc. The system could be a relatively hard and X-ray luminous polar or an intermediate polar, possibly nearly synchronous.

The REgolith X-Ray Imaging Spectrometer (REXIS) for OSIRIS-REx: Identifying Regional Elemental Enrichment on Asteroids

The OSIRIS-REx Mission was selected under the NASA New Frontiers program and is scheduled for launch in September of 2016 for a rendezvous with, and collection of a sample from the surface of asteroid Bennu in 2019. 101955 Bennu (previously 1999 RQ36) is an Apollo (near-Earth) asteroid originally discovered by the LINEAR project in 1999 which has since been classified as a potentially hazardous near-Earth object. The REgolith X-Ray Imaging Spectrometer (REXIS) was proposed jointly by MIT and Harvard and was subsequently accepted as a student led instrument for the determination of the elemental composition of the asteroids surface as well as the surface distribution of select elements through solar induced X-ray fluorescence. REXIS consists of a detector plane that contains 4 X-ray CCDs integrated into a wide field coded aperture telescope with a focal length of 20 em for the detection of regions with enhanced abundance in key elements at 50 m scales. Elemental surface distributions of approximately 50-200 m scales can be detected using the instrument as a simple collimator. An overview of the observation strategy of the REXIS instrument and expected performance are presented here.

Development of the ProtoEXIST2 advanced CZT detector plane

The ProtoEXIST program was conceived for the development of a highly scalable detector plane architecture utilizing pixilated CdZnTe (CZT) detectors for eventual deployment in a large scale (1–4 m2 active area) coded aperture X-ray telescope. Development is now underway for ProtoEXIST2, which ultimately will be comprised of a closely tiled 8×8 array of 19.9 mm × 19.9 mm, 5 mm thick Redlen CZT crystals, analogous to ProtoEXIST1, but will now utilize the NuASIC which accommodates the direct bonding of CZT detectors with a 32×32 pixilated anode with a 604.8 µm pixel pitch and a guard ring surrounding the perimeter of the anode pattern. Currently 6 prototype detectors have been fabricated and tested. An energy threshold of approximately 6 keV and energy resolutions of 2.5 keV FWHM at 122.0 keV and 2.1 keV at 59.6 keV have been achieved with individual detectors. The performance and energy resolution of the individual prototype detectors is discussed in detail. In addition, during preliminary testing of the prototype ProtoEXIST2 detectors reduced photopeak efficiency and pixel distortions were observed for pixels within approximately 3 mm of the CZT edge. These distortions have been largely corrected with two independent techniques: the application of an 73 mm × 73 mm extended cathode centered directly above the detector and the use of a grounded, 6mm tall, electrostatic side shield surrounding the perimeter of the detector. The effects of the side shield and extended cathode are covered in detail.

ProtoEXIST: advanced prototype CZT coded aperture telescopes for EXIST

ProtoEXIST1 is a pathfinder for the EXIST-HET, a coded aperture hard X-ray telescope with a 4.5 m2 CZT detector plane a 90x70 degree field of view to be flown as the primary instrument on the EXIST mission and is intended to monitor the full sky every 3 h in an effort to locate GRBs and other high energy transients. ProtoEXIST1 consists of a 256 cm2 tiled CZT detector plane containing 4096 pixels composed of an 8x8 array of individual 1.95 cm x 1.95 cm x 0.5 cm CZT detector modules each with a 8 x 8 pixilated anode configured as a coded aperture telescope with a fully coded 10° x 10° field of view employing passive side shielding and an active CsI anti-coincidence rear shield, recently completed its maiden flight out of Ft. Sumner, NM on the 9th of October 2009. During the duration of its 6 hour flight on-board calibration of the detector plane was carried out utilizing a single tagged 198.8 nCi Am-241 source along with the simultaneous measurement of the background spectrum and an observation of Cygnus X-1. Here we recount the events of the flight and report on the detector performance in a near space environment. We also briefly discuss ProtoEXIST2: the next stage of detector development which employs the NuSTAR ASIC enabling finer (32×32) anode pixilation. When completed ProtoEXIST2 will consist of a 256 cm2 tiled array and be flown simultaneously with the ProtoEXIST1 telescope.

ERRATUM: “MILAGRO OBSERVATIONS OF MULTI-TeV EMISSION FROM GALACTIC SOURCES IN THE FERMI BRIGHT SOURCE LIST” (2009, ApJ, 700, L127)

The position of the peak of the Milagro excess coincident with 0FGL J2229.0+6114 was incorrectly reported as R.A. = 22h28m17s, decl. = 60°29m. The correct position is R.A. = 22h28m44s, decl. = 61°10m with a statistical position error of 0165.

Imaging and burst location with the EXIST high-energy telescope

The primary instrument of the proposed EXIST mission is a coded mask high energy telescope (the HET), that must have a wide field of view and extremely good sensitivity. In order to achieve the performance goals it will be crucial to minimize systematic errors so that even for very long total integration times the imaging performance is close to the statistical photon limit. There is also a requirement to be able to reconstruct images on-board in near real time in order to detect and localize gamma-ray bursts, as is currently being done by the BAT instrument on Swift. However for EXIST this must be done while the spacecraft is continuously scanning the sky. The scanning provides all-sky coverage and is also a key part of the strategy to reduce systematic errors. The on-board computational problem is made even more challenging for EXIST by the very large number of detector pixels (more than 107, compared with 32768 for BAT). The EXIST HET Imaging Technical Working Group has investigated and compared numerous alternative designs for the HET. The selected baseline concept meets all of the scientific requirements, while being compatible with spacecraft and launch constraints and with those imposed by the infra-red and soft X-ray telescopes that constitute the other key parts of the payload. The approach adopted depends on a unique coded mask with two spatial scales. Coarse elements in the mask are effective over the entire energy band of the instrument and are used to initially locate gamma-ray bursts. A finer mask component provides the good angular resolution needed to refine the burst position and reduces the cosmic X-ray background; it is optimized for operation at low energies and becomes transparent in the upper part of the energy band where an open fraction of 50% is optimal. Monte Carlo simulations and analytic analysis techniques have been used to demonstrate the capabilities of the proposed design and of the two-step burst localization procedure.

The high resolution X-ray imaging detector planes for the MIRAX mission

The MIRAX X-ray observatory, the first Brazilian-led astrophysics space mission, is designed to perform an unprecedented wide-field, wide-band hard X-ray (5?200 keV) survey of Galactic X-ray transient sources. In the current configuration, MIRAX will carry a set of four coded-masks telescopes with high spatial resolution Cadmium Zinc Telluride (CZT) detector planes, each one consisting of an array of 64 closely tiled CZT pixelated detectors. Taken together, the four telescopes will have a total detection area of 959 cm2, a large field of view (60? ? 60? FWHM), high angular resolution for this energy range (6 arcmin) and very good spectral resolution ( ~ 2 keV @ 60 keV). A stratospheric balloon-borne prototype of one of the MIRAX telescopes has been developed, tested and flown by the Harvard-Smithsonian Center for Astrophysics (CfA) as part of the ProtoEXIST program. In this paper we show results of validation and calibration tests with individual CZT detectors of the ProtoEXIST second generation experiment (P2). Each one of 64 detector units of the P2 detector plane consists of an ASIC, developed by Caltech for the NuSTAR telescope, hybridized to a CZT crystal with 0.6 mm pixel size. The performance of each detector was evaluated using radioactive sources in the laboratory. The calibration results show that the P2 detectors have average energy resolution of ~ 2.1 keV @ 60 keV and 2.3 @ 122 keV. P2 was also successfully tested on near-space environment on a balloon flight, demonstrating the detector unit readiness for integration on a space mission telescope, as well as satisfying all MIRAX mission requirements.

The EXIST optical and infra-red telescope (IRT) and imager-spectrometer

The EXIST (Energetic X-ray Imaging Survey Telescope) mission includes the 1.1 m optical Infra-Red Telescope (IRT) which provides the capability to locate, identify, and obtain spectra of transient events, in particular GRB afterglows at redshifts up to epoch of reionization. The instrument includes a high spatial resolution imager, low spectral resolution spectrometer (R~ 30) and high resolution slit spectrometer (R~ 3000). This instrument, with the observatorys rapid reaction response will quickly identify the GRB afterglow, measure its brightness curves, redshift, measure spectral characteristics of the afterglows and measure absorption spectra of the intervening intergalactic medium. With this instrument, high redshift GRBs become important tools for studying the growth of structure, observing the processes through which the universe is reionized.

Efficient validation testing of Through-Silicon-Via (TSV) ASICs for CZT x-ray detectors

The High Resolution Energetic X-ray Imager (HREXI ) is a coded-aperture imaging telescope that utilizes a large closely-tiled array of CdZnTe (CZT) detectors, each 19.9 x 19.9 x 3mm with a 32 x 32 pixel (604μm) for coded aperture X-ray imaging (3 - 200 keV) of cosmic X-ray sources and transients. Each CZT crystal is read out by an ASIC incorporating, for the first time, Through Silicon Vias (TSVs). These TSVs replace the wire bonds for this ASIC, originally designed for the Nuclear Spectroscopic Telescope Array (NuSTAR) focusing hard X-ray telescope. The TSVs allow flip-chip bonding of the ASIC to the PCB board electronics for processing of the data. The new TSV-ASICs will enable closer tiling and larger imaging arrays which require faster, more efficient ASIC testing and calibration at the die level. We have designed and developed an ASIC Test Stand (ATS) for rapid ASIC testing prior to bonding to CZT. We demonstrate how ASIC die-level testing with the ATS can be performed rapidly with rigidly spaced micro-pogo pins supported by an FPGA readout.